This competing continuation grant focusses on alloreactivity mediated by CD4+ and CD8+ T- cells in mice. This is an important topic because T-cell responses to alloantigens are the primary cause of graft-versus-host disease (GVHD), graft rejection and other forms of transplantation immunity. Two broad lines of investigation are proposed. Alloreactivity by CD4+ cells. To study the effector functions of CD4+ cells in vivo, information will be sought with a model in which purified CD4+ cells transferred to irradiated MHC-class-II-different hosts without donor stem cells cause bone marrow (BM) aplasia and death from anemia. Preliminary evidence suggests that CD4+ cells induce BM aplasia by causing direct MHC- restricted destruction of stem cells. This idea will be tested by examining the role of perforins versus soluble mediators, the possible involvement of Fas and the range of cell types attacked by CD4+ cells. Tolerance induction of mature CD4+ cells will be explored with a model in which irradiated class II-different hosts are reconstituted with a mixture of donor CD4+ cells and donor stem cells. Using an allotype difference between these cell types, tolerance induction at the level of mature and newly-formed CD4+ cells will be studied. Preliminary work has shown that the initial GVH reaction by the mature donor CD4+ cells impairs tolerance induction of newly-formed CD4+ cells, perhaps by inducing thymic damage. This possibility will be assessed. Alloreactivity by CD8+ cells. CD8+ T-cells prepared from T-cell receptor transgenic mice will be used as a tool for defining the specificity, triggering requirements and effector functions of CD8+ cells. In vitro systems involving the use of defined peptides as antigens and well- characterized cell lines as antigen presenting cells will be employed to establish the cellular signals needed to stimulate unprimed CD8+ cells to respond to defined alloantigens. Alloreactive responses of CD8+ cells will also be studied under in vivo conditions, using GVHD as a model.